According to Google's head of web spam team, Matt Cutts, Guest posting is not useful anymore. Here's their official take https://www.mattcutts.com/blog/guest-blogging/ .

I think forum posting and blog commenting is still effective, the only problem is, people don't often do it the right way. Let me share one thing about forum posting. If you join a forum related to your product and you post a lot of messages (relevant and useful only) and your account is not brand new, it has passed few weeks or months, then if you post a backlink to your website (at an appropriate place and in an appropriate way) then neither forum moderators would mind it, nor would Google. And if you just joined the forum today and you write a signature like this "Hi this is very good forum. Thanks. www.BlahBlahBlah....com then of course it's more than a spam.

Do you people know, last month, I was quite nervous, that Google says everything is spam. So how we should build natural links! Then I did my research, I didn't work for 2 months that time, since I wanted to discover something new, then I figured out the natural way to create Google friendly backlinks.

With those, we can compete with big brands in any industry as well.